Heat olive oil in a large pot over medium heat. Add onion, carrots, celery, parsnip, and yellow bell pepper. Cook 5–7 minutes until softened.
Stir in garlic, thyme, and rosemary. Cook for 1–2 minutes until fragrant.
Add vegetable broth, miso paste, and bay leaf. Bring to a boil, then reduce heat and simmer for 15–20 minutes until vegetables are tender.
Remove half the vegetables and set aside. Blend remaining soup until smooth with an immersion blender (or carefully in a blender). Return reserved vegetables.
Stir in cooked pastina. Add lemon juice and parsley. Season with salt and pepper to taste.